1. Field of the Invention
The present invention relates to a data transfer control system, and more particularly to a data transfer control in a system that realizes data multiplication by copying data among storage systems.
2. Related Background Art
In recent years, the necessity of disaster recovery systems has been rising for computer systems in order to avoid data loss in the event of a disaster such as terrorism and earthquake. To this end, data from a host is generally written to a master storage as well as to a remote storage installed at a remote location, thereby realizing data duplication (or data multiplication in a broader sense). As a result, even when a master system cannot be operated due to a disaster, operations can be resumed shortly by switching to a remote system.
In the duplication system described, a remote copy is meaningful only when the remote system is operating normally and a backup copy to the remote storage can be made at all times. A failure of equipment between storage systems or an instruction from a user, for example, can make it temporarily impossible to transfer data between storage systems, which results in a suspension of data duplication. Such a state is called a “suspended state.” Even during the suspended state, write I/O data from the host continues to be written to storage media of the master storage and data is updated constantly. Updates to data written to the storage media of the master storage are managed by a so-called “differential bitmap,” which expresses all storage regions of the storage media in a bitmap format and flags bits that correspond to updated regions (i.e., differential data regions).
When the equipment recovers from the failure, the suspended state is canceled, and when the remote system returns to its normal state, a processing to transfer differential data indicated on the differential bitmap from the master storage to the remote storage and recover the remote storage to a completely mirroring state (i.e., a recovery copy processing) takes place.
When a major failure occurs in the master system during the recovery copy processing, a situation in which data consistency cannot be maintained even in the remote system may occur.
A technology to solve such a problem has been proposed. When executing a recovery copy processing (in other words, when a recovery copy mode begins), before sending differential data from a master system to a remote system, old data in disk drive devices that corresponds to the differential data is saved to a different region of a spare disk; by subsequently storing the differential data to a target storage region of the disk drive devices, the consistency of the copy data is maintained.
However, when the master system becomes unusable for whatever reason, the conventional technology requires some waiting time until the recovery copy processing ends, which requires some time before system operations can be resumed.
Furthermore, when there is a large amount of write I/O data sent to the master storage and when the master storage and the remote storage are far apart or when there is insufficient band in a data transfer line, the response time to the host for the large amount of write I/O data can deteriorate considerably in data duplication state, regardless of whether the data transfer to the remote storage is synchronous or asynchronous.